The coaptation of the bone fragments using plates of titanium or another material and screws in order to create an osteosynthesis is a common operation in bone surgery, for example, orthopedic surgery.
In order to obtain a good result, it is necessary that the plates or implants are screwed together permanently to the bone fragments assembled from the implants. It is thus a requirement that the screws can not unscrew and move back, to prevent any displacement of the implants relative to the bone fragments.
On the other hand, it would often be desirable to be able to choose the orientation of the screws relative to the plates and as a function of the positioning and the shape of the fragments to be assembled, which can improve the quality of the assembly.
In order to prevent the screws from unscrewing and moving back, it has been proposed (EP0,345,133, FR-2,794,963) to house the locking instruments at the input of the screw holes into the plates, in order to eliminate any possibility of axial movement of the screws, after they are tightened in the bony material. For example, it is provided in the document EP-0,345,133, to use check screws that are outside-threaded and work together with a complementary threading arranged at the input of the holes of the screws that equip the plates, in a manner so that the head of the screws is wedged against a check screw and that the check screws can not move axially relative to the plates, this locking thus ensuring the permanent support of the plate on the bone fragments.
The devices proposed by several manufacturers represent at this time the solutions that are most certain in terms of locking. However, these relatively complex devices require the use of plates having a relatively sizeable thickness that is totally incompatible with a usage for operations on the bones of the hand or foot, for which the thickness of the plates must be as reduced as possible, taking into account the small size of the bones involved.
In the document EP-0.345.133, a device is again shown for connecting two elements such as an implant and a bone, according to which the implant comprises screw holes with axes oriented at an angle to each other, in a manner so that the screws going through these holes have orientations that are rigorously imposed by the direction of these axes. A device of this type can only be planned for the reduction of identical fractures, because otherwise it would be necessary to provide as many plate models as possible cases of fractures, which is practically impossible; in fact, it does not offer any possibility to choose the orientation of the screws as a function of the problems encountered in orthopedic surgery.
In the document WO-00/66012, a plate is described for osteosynthesis that can be locked, according to which the screws and the screw holes provided in the plate are equipped, respectively, with a locking threading and a sensible meshing profile allowing the introduction of the screws into the plate in an angled manner. The practical creation of a device of this type would be difficult and it seems its effectiveness has not been established.
In a general manner, in the field of osteosynthesis of small bones requiring the use of plates having small dimensions, the devices that are currently available on the market do not make possible an angular clearance between the screws and the plate, so that the screws thus have to be positioned perpendicularly to the plate. However, in certain cases, it would be desirable to be able to have one or more screws at an angle in order to use one or more of the better quality bones for tightening the screws.